Lubricated metallic container stocks and method of preparing the same and applying organic coating thereto

ABSTRACT

Metallic container stocks are lubricated with citric acid esters. The improved lubricated container stocks of the invention are characterized by excellent wettability and adhesion of organic coatings in general and especially epoxy resin coating compositions. The citric acid ester lubricants are pharmacologically safe and are easily applied at low cost by electrostatic deposition and other methods presently used for applying prior art lubricating agents. The method of the invention is especially useful in lubricating tinplate strip, blackplate strip, blackplate strip having a chromium-containing coating thereon, and aluminum strip of container stock gauge with citric acid ester lubricants having the following structural formula: ##STR1## wherein R 1 , R 2  and R 3  are individually selected from the group consisting of hydrogen and alcoholic residual containing 1-18 carbon atoms, R 4  is selected from the group consisting of hydrogen and carboxylic acid radicals containing 1-18 carbon atoms, and at least one of R 1 , R 2  and R 3  is an alcoholic residue. The invention further provides a novel method of applying an organic coating on lubricated metallic container stocks.

BACKGROUND OF THE INVENTION

The present invention broadly relates to improved lubricated metalliccontainer stocks and to a novel method of preparing the same. In stillanother of its aspects, the invention further relates to a novel methodof applying an organic coating on lubricated metallic container stocks.

A thin coating of a lubricant is applied on the surfaces of mostmetallic container stocks produced in this country. The lubricant isnecessary in order to facilitate handling of sheets of the product andto protect the surfaces against abrasion, scratches, and to some extentcorrosion. If the lubricant is not applied, the individual sheets tendto stick together and do not handle well in subsequent fabricating stepsutilizing automatic machines for applying organic coatings,lithographing and manufacturing containers.

The amount of lubricant that is applied is quite small and must bemaintained within a relatively narrow critical range. If too littlelubricant is applied, then the disadvantages enumerated above when nolubricant is applied are present to some extent. If too much lubricantis applied, there is excessive slippage and it is difficult to stack andhandle the individual sheets. Excess oil may also adversely affect theapplication of organic coatings, lithographing, labeling and othersubsequent steps. The lubricant usually should be applied in an amountof about 0.05-1.0 gram per base box. The term "base box" is used in thisart to refer to a quantity of metallic container stock having a totalsurface area, i.e., the surface area of both sides, of 62,720 squareinches. The term "base box" is used hereinafter for convenience indisclosing and claiming the invention and refers to a total surface areaof 62,720 square inches on which the lubricant is applied.

Lubricants may be applied to metallic container stock by a number ofmethods such as branning, dipping in a solvent solution of the lubricantfollowed by evaporating the solvent, and electrostatic deposition.Electrostatic deposition is a method that is suitable for theapplication of the small quantities of lubricant set out above in theform of a thin uniform film at high line speeds. The line speedsemployed in the manufacture of metallic container stocks sometimes varyfrom 250 feet per minute to as high as 2,000 feet per minute and amethod capable of control of the rate of lubricant application for theseline speed variations is required. As a result, the lubricant that isselected for commercial use should be capable of being applied byelectrostatic deposition.

A lubricant for lubricating metallic container stocks should possess aunique combination of properties. If any one of this combination ofproperties is missing, then the lubricant is not entirely satisfactory.As a result, only a very small percentage of the numerous potentiallubricants are satisfactory. The problem of selecting a suitablelubricant is complicated by the fact that it is often impossible topredict whether or not a specific substance will be suitable bylaboratory scale lubricating experiments and the potential lubricantmust be applied in a production run.

Some of the more important characteristics of an entirely satisfactorylubricant for metallic container stocks are as follows:

(1) In the case of electrostatic deposition, the lubricant should beeasily atomized to form a finely divided dispersion thereof in a gaseousmedium such as air. A carefully controlled quantity of the suspendedparticles of lubricant in the resultant gaseous dispersion should becapable of being electrostatically precipitated in the form of a thinuniform film upon metallic container stock moving at high line speeds.

(2) The lubricant should be retained on the container stock for areasonable period of time in an amount effective to lubricate thesurface.

(3) The lubricant should be compatible with protective and/or decorativeorganic coatings which are subsequently applied to the lubricatedcontainer stock. The lubricant also should have no effect or abeneficial effect on the wetting of the surface area to be coated andthe adhesion thereto of the protective and/or decorative organiccoatings.

(4) The lubricant should not be discolored upon heating or baking thelubricated container stock at a temperature sufficiently high to hardenor cure the organic coatings.

(5) The film of lubricant should be resistant to oxidation and notharden or lose its desirable lubricant properties over a reasonableperiod of time.

(6) The lubricant should be free of objectionable tastes and odors inthe quantities applied and under the conditions of use.

(7) The lubricant must be pharmacologically safe and nontoxic in theamounts applied as the lubricated container stocks are often used tomanufacture containers for preserving and storing foodstuffs.

A number of naturally occurring vegetable oils and synthetic esters ofcarboxylic acids have been proposed heretofore for use in lubricatingmetallic container stocks. Vegetable oils such as cotton seed oil, palmoil and the like have a tendency to oxidize to a solid film which is nolonger a good lubricant after a relatively short period of storage. Anumber of the synthetic ester lubricants have this deficiency, and as aresult of this and other advantages, several of the esters of sebacicacid have been used heretofore as lubricants. Synthetic esters ofsebacic acid prepared from alcohols containing eight or more carbonatoms, such as dioctyl sebacate, are widely used as lubricants at thepresent time. While dioctyl sebacate is one of the best synthetic esterlubricants available heretofore, it does have some deficiencies in viewof recent developments in the art. For example, a substantial percentageof the metallic container stock produced in this country is given acathodic dichromate treatment such as disclosed in U.S. Pats. No.3,278,401 and 3,296,106. The cathodic dichromate treatment increasescorrosion resistance and is desirable for this purpose. When thecathodic dichromate treated container stock is lubricated with dioctylsebacate or other prior art lubricants, wetting or eyeholing problemsoften occur. This tendency is especially pronounced when epoxy typelacquers and certain other advanced organic coatings are used. As aresult of this deficiency, the art has long sought a suitable lubricantwhich meets all of the requirements set out above, regardless of thetype of organic coating and whether or not the container stock hasreceived a cathodic dichromate treatment. However, an entirelysatisfactory lubricant was not available prior to the present invention.

It is an object of the present invention to provide an improvedlubricated metallic container stock which is readily wetted by initiallyfluid organic coating materials which often present wetting problemssuch as epoxy lacquers.

It is a further object to provide a novel method of lubricating metalliccontainer stocks wherein the lubricant is an ester of citric acid.

It is still a further object to provide a novel method of applying anorganic coating on lubricated container stocks wherein the lubricatedsurface area to be coated is easily wetted by initially fluid organiccoating materials and whereby eyeholing and other imperfections in thefinal hardened organic coating may be minimized.

Still other objects and advantages of the present invention will beapparent to those skilled in the art upon reference to the followingdetailed description.

DETAILED DESCRIPTION OF THE INVENTION INCLUDING PREFERRED EMBODIMENTSAND VARIANTS THEREOF

The present invention provides improved metallic container stock havinga thin substantially uniform film of a citric acid ester lubricant on atleast one surface area thereof. The citric acid ester lubricant ispresent in an amount to lubricate the surface area and preferably ispresent in an amount of about 0.05-1.0 gram per base box, i.e., about0.05-1.0 gram for each 62,720 square inches of lubricated surface area.In instances where the lubricated container stock is to receive anorganic coating in a subsequent processing step, then usually betterresults are obtained when citric acid ester lubricant is present in anamount of about 0.15-0.40 gram per base box.

Citric acid ester lubricants for use in practicing the invention mayhave the following structural formula: ##STR2## wherein R₁, R₂ and R₃are individually selected from the group consisting of hydrogen andalcoholic residua containing 1-18 carbon atoms, R₄ is selected from thegroup consisting of hydrogen and carboxylic acid radicals containing1-18 carbon atoms, and at least one of R₁, R₂ and R₃ is an alcoholicresidue. In some instances, it is prefered that R₁, R₂ and/or R₃ bealcoholic residua containing 1-10 and often for better results 1-4carbon atoms, and/or that R₄ be selected from the group consisting ofhydrogen and carboxylic acid radicals containing 1-10 and often forbetter results 1-4 carbon atoms. In other instances, it is preferredthat each of R₁, R₂ and R₃ be alcoholic residua containing 1-10 andpreferably 1-4 carbon atoms, and/or that R₄ be either hydrogen or acarboxylic acid radical containing 1-10 and preferably 1-4 carbon atoms.

Specific examples of presently preferred citric acid ester lubricantsinclude triethyl citrate, acetyl triethyl citrate, tributyl citrate,acetyl tributyl citrate, acetyl tri-2-ethylhexyl citrate, and admixturesof one or more thereof. Acetyl tributyl citrate often gives the bestresults.

Any suitable prior art metallic container stock may be lubricated withthe citric acid ester lubricants of the invention. Usually tinplate,blackplate, blackplate having a chromium-containing coating thereon andmetallic aluminum of container stock gauge are the preferred containerstocks. Metallic container stocks are usually in the form of sheet orcontinuous strip, but may be in other shapes convenient for use in themanufacture of containers. Container stocks suitable for use inpracticing the invention are disclosed in numerous texts and U.S.patents. For example, blackplate and tinplate are disclosed in "TheMaking, Shaping and Treating of Steel," 7th edition, published by theUnited States Steel Corporation and aluminum container stocks aredisclosed in the text "Aluminum," published by the American Society forMetals, of which chaper 23 in volume 3, is the most pertinent.Blackplate having a chromium-containing coating thereon is disclosed inPats. No. 3,316,160, 3,526,486, 3,567,599, and 3,475,295. Pat. No.3,526,486 discloses blackplate having a metallic chromium-containingcoating thereon contiguous with the ferrous metal surface and anonmetallic chromium-containing film thereover which is cathodicallydeposited from an aqueous hexavalent chromium electrolyte. Thedisclosures in the above texts and patents are incorporated herein byreference.

The metallic container stocks to be lubricated may be given anelectrochemical treatment in an aqueous electrolyte containing a watersoluble chromium compound to deposit a chromium-containing film thereonwhich increases corrosion resistance. U.S. Pat. No. 3,278,401 disclosesa method of electrochemically depositing a chromium-containing film ontinplate, which preferably but not necessarily is flow brightenedelectrolytic tinplate. The chromium-containing film is cathodicallydeposited on the tinplate from an aqueous hexavalent chromiumelectrolyte. Cathodic dichromate treatment is usually preferred. Pat.No. 3,296,106 discloses the treatment of various metallic substrates toelectrochemically deposit thereon a chromium-containing film whichincreases corrosion resistance. The metallic substrates are cathodicallytreated in an aqueous hexavalent chromium electrolyte and the treatmentis effective for blackplate, iron tin alloy coated ferrous metal,tinplate and other metallic container stocks. The deposition of anonmetallic chromium-containing film on blackplate coated with metallicchromium is disclosed in Pat. No. 3,526,486. The teachings of the abovepatents are likewise incorporated herein by reference. It is understoodthat the metallic container stocks may be given an electrochemicaltreatment such as described therein, or other prior art treatments, forthe purpose of increasing corrosion resistance or for imparting otherdesirable properties prior to applying the lubricant.

The products produced in accordince with the teachings of U.S. Pats. No.3,278,401 and 3,526,486 are especially desirable metallic containerstocks and thus are often preferred. The citric acid ester lubriants areespecially effective with these substrates.

Any suitable prior art method for lubricating metallic container stockmay be used for applying the citric acid ester lubricant in the form ofa thin substantially uniform film. Examples of prior art methods includeelectrostatic deposition, branning, applying a solution of the lubricantin a volatile solvent followed by evaporating the solvent, applying anaqueous emulsion of the lubricant followed by evaporating the water, andapplying the lubricant per se directly to the container stock by meansof metering rolls, spraying or the like when it is sufficiently fluid.Electrostatic deposition is usually the preferred method of applicationin the commercial production of metallic container stocks. The citricacid esters described herein are easily applied by electrostaticdeposition and have the desirable characteristics enumeratedhereinbefore. Additionally, the synthetic citric acid esters enhancewettability of the lubricated surface by the initially fluid organiccoating and reduce surface imperfections such as eyeholing. Adhesion isalso improved and the final hardened or cured organic coatings producedin accordance with the coating method of the invention exhibit excellentadherence. These beneficial effects extend to container stocks whichhave received an electrochemical treatment prior to applying thelubricant. Cathodic dichromate treatment of tinplate and other metallicsubstrates often tends to introduce wetting problems. This problem maybe eliminated by using the citric acid ester lubricants.

In practicing the organic coating method of the present invention, anysuitable prior art fluid organic coating material may be used such asvarnishes, lacquers and enamels for metallic container stocks. Specificexamples of organic coating materials include epoxy, modified epoxy,phenolic, modified phenolic, acrylic, modified acrylic, vinyl, modifiedvinyl, alkyd and polyurethane varnishes, lacquers and/or enamels. Priorart lithographing or printing inks which are suitable for use onmetallic container stocks for labeling and decorative purposes areconsidered to be organic coatings for the purpose of the presentinvention. The organic coating materials should be fluid initially,i.e., sufficiently fluid to be applied by metering rolls, spraying,brushing, and direct or indirect printing or lithographing; and may varyfrom soft pastes to fluid liquids depending upon the selected method ofapplication. The initially fluid organic coating material is hardened orcured following application by, for example, evaporating solvent orbaking to produce the final organic coating. The method of applying theinitially fluid organic coating material and then hardening the same toproduce the final organic coating may be in accordance with prior arepractice. However, the metallic container stock to be coated must belubricated with a citric acid ester of the present invention. The citricacid ester lubricant assures substantially instantaneous wetting of thesurface by the initially fluid organic coating material and allows highspeed coating lines to be employed with all organic coating materialsincluding those which tend toward poor wetting properties. Additionally,the hardened or cured organic coating tends to adhere more tightly tothe citric acid ester lubricated surface and any tendency toward pooradherence is overcome and eyeholing or other imperfections are not aproblem. Thus, the present invention overcomes the problems of the priorart when operating high speed coating lines and especially when usingorganic coating compositions which normally present wetting problems.

The foregoing detailed description of the invention and the followingspecific examples are for purposes of illustration only and are notintended as being limiting to the spirit or scope of the appendedclaims.

EXAMPLE I

Tinplated strip delivered from a prior art electrolytic tinplating lineat a strip speed above 1,000 feet per minute is passed continuouslythrough a flow brightening zone where it is flow brightened, followed byquenching with water and a cathodic dichromate treatment in accordancewith U.S. Pat. No. 3,278,401. The resulting cathodic dichromate treatedflow brightened tinplate is rinsed, dried and passed through anelectrostatic lubricating zone at a speed corresponding to the speed ofthe tinplating line. Acetyl tributyl citrate was atomized to form afinely divided gaseous suspension, and the gaseous suspension was passedthrough an ionizing zone where the suspended particles of acetyltributyl citrate lubricant were given an electrical charge. A stream ofthe gaseous suspension of ionized lubricant particles was directedtoward and electrostatically precipitated on the electrically groundedmoving strip in the form of a thin uniform film. The lubricated tinplatewithdrawn from the electrostatic lubricating zone had 0.26 gram per basebox of acetyl tributyl citrate on the surface.

The lubricated flow brightened tinplated strip was sheared into sheets.The lubricated tinplate sheets handled well in automatic equipmentwithout undue abrasion or scratching and the product was used in theExamples appearing hereinafter.

EXAMPLE II

A second run was made in accordance with Example I with the exception ofsubstituting dioctyl sebacate as the lubricant for the acetyl tributylcitrate. The resulting product had 0.27 gram per base box of dioctylsebacate lubricant on the sheet surfaces.

EXAMPLE III

Samples of lubricated tinplate from Example I and Example II were storedfor four months under identical conditions to determine the amount oflubricant lost during the period of storage.

The product of Example I initially had 0.26 gram per base box of acetyltributyl citrate on its surface, and after four months of storage 0.20gram per base box. The product of Example II initially had 0.27 gram perbase box of dioctyl sebacate on its surface, and only 0.17 gram per basebox after four months of storage. It is therefore apparent that the rateof loss of acetyl tributyl citrate during storage is substantially lessthan that of dioctyl sebacate. This is an unexpectedly low rate of lossof acetyl tributyl citrate since dioctyl sebacate is considered to bevery outstanding in this respect.

EXAMPLE IV

Samples of lubricated tinplate from Example I and from Example II weretested under identical conditions with diluted lacquers to determine theeffect of the two lubricants on wetting. One part of the lacquer wasmixed with six parts of solvent and one drop of the resulting mixturewas applied to the surface of a tinplate sample from each of Examples Iand II laid horizontally on a flat level surface. The drops of dilutedlacquer were allowed to dry in air, and then baked for a permanentrecord. The area of flow of each system was measured in square inchesand the results recorded.

In one run, one part by volume of epoxy lacquer was diluted with sixvolumes of isophorone and the mixture was tested as set out above. Thismixture gave an area of flow of 0.24 square inch for the dioctylsebacate oiled tinplate, and 2.4 square inches for the acetyl tributylcitrate oiled tinplate.

In another run, one part of epoxy lacquer was diluted with six parts ofdiacetone alcohol and the mixture was tested as set out above. Thedioctyl sebacate oiled tinplate had an area of flow of 0.17 square inchand the acetyl tributyl citrate oiled tinplate had an area of flow of0.95 square inch.

In a further run, one part of epoxy lacquer was diluted with six volumesof ethylene glycol monoethyl ether acetate and the mixture was tested asset out above. The dioctyl sebacate oiled tinplate had an area of flowof 0.62 square inch, and the acetyl tributyl citrate oiled tinplate hadan area of flow of 2.1 square inches.

In a further run, one part of epoxy lacquer was diluted with six partsof ethylene glycol monoethyl ether and the mixture was tested as set outabove. The dioctyl sebacate oiled tinplate had an area of flow of 0.6square inch, and the acetyl tributyl citrate oiled tinplate had an areaof flow of 1.9 square inches.

It is clear from the above data that the acetyl tributyl citrate oiledtinplate was much more easily wetted by the various diluted epoxylacquers than the dioctyl sebacate oiled tinplate.

EXAMPLE V

Individual runs were made using the lubricated product from Example Iand the lubricated product from Example II in a high speed line forlacquering and lithographing the sheets. The organic coating, which wasused in both runs, was an epoxy lacquer which exhibited a tendencytoward eyeholding and poor wetting.

No problems were experienced with respect to eyeholing and wetting whenthe lubricated product of Example I, i.e., the acetyl tributyl citratelubricated tinplate was being run. However, the lubricated product ofExample II, i.e., the dioctyl sebacate lubricated tinplate exhibitedeyeholding and wetting problems and was not satisfactory in theserespects. The hardened film of epoxy lacquer on the product of Example Iwas also more adherent than on the product of Example II.

Upon making further runs identical with those of Examples I through Vabove with respect to the oiling step, except for substitutingblackplate, blackplate having a chromium-containing coating and aluminumsheet for the tinplate of Examples I through V in the oiling step,comparable results are obtained. In each instance, the acetyl tributylcitrate lubricated container stock exhibits better wetting propertiesand the hardened organic coating films applied thereto are more adherentthan when dioctyl sebacate is the lubricant.

We claim: .[.1. A metalic container stock having a thin substantiallyuniform film of a citric acid ester lubricant on at least one surfacearea thereof, the said citric acid ester lubricant consistingessentially of at least one citric acid ester of an alcohol containing1-10 carbon atoms and being present in an amount of about 0.05-1.0 gramfor each 62,720 square inches of lubricated surface area..]. .[.2. Thelubricated container stock of claim 1 wherein the citric acid esterlubricant is present in an amount of about 0.15-0.40 gram for each62,720 square inches of lubricated surface area..]. .[.3. The lubricatedcontainer stock of claim 1 wherein the container stock is selected fromthe group consisting tinplate, blackplate, blackplate having achromium-containing coating thereon, and metallic aluminum of containerstock gauge..]. .[.4. The lubricated container stock of claim 3 whereinthe container stock is blackplate..]. .[.5. The lubricated containerstock of claim 3 wherein the container stock is blackplate having achromium-containing coating thereon..]. .[.6. The lubricated containerstock of claim 3 wherein the container stock is metallic aluminum ofcontainer stock gauge..]. .[.7. The lubricated container stock of claim3 wherein the container stock is tinplate..]. .[.8. The lubricatedcontainer stock of claim 1 wherein the said film consists essentially ofat least one citric acid ester lubricant having the structural formula:##STR3## wherein R₁, R₂ and R₃ are individually selected from the groupconsisting of hydrogen and alcoholic residua containing 1-10 carbonatoms, R₄ is selected from the group consisting of hydrogen andcarboxylic acid radicals containing 1-10 carbon atoms, and at least oneof R₁, R₂ and R₃ is an alcoholic residue..]. .[.9. The lubricatedcontainer stock of claim 8 wherein R₁, R₂ and R₃ are alcoholic residuacontaining 1-4 carbon atoms..]. .[.10. The lubricated container stock ofclaim 8 wherein R₁, R₂ and R₃ are alcoholic residua containing 1-4carbon atoms, and R₄ is selected from the group consisting of hydrogenand carboxylic acid radicals containing 1-4 carbon atoms..]. .[.11. Thelubricated container stock of claim 8 wherein the citric acid esterlubricant consists essentially of at least one substance selected fromthe group consisting of triethyl citrate, acetyl triethyl citrate,tributyl citrate, acetyl tributyl citrate, acetyl tri-2-ethylhexylcitrate, and admixtures thereof..]. .[.12. The lubricated containerstack of claim 8 wherein the citric acid ester lubricant consistsessentially of acetyl tributyl citrate..]. .[.13. The lubricatedcontainer stack of claim 8 wherein the citric acid ester consistsessentially of acetyl tributyl citrate and is applied in an amount ofabout 0.15-0.40 gram for each 62,720 square inches of lubricated surfacearea..]. .[.14. The lubricated container stock of claim 3 wherein thecontainer stock is tinplate and a film comprising at least one citricacid ester lubricant having the structural formula: ##STR4## wherein R₁,R₂ and R₃ are individually selected from the group consisting ofhydrogen and alcoholic residua containing 1-10 carbon atoms, R₄ isselected from the group consisting of hydrogen and carboxylic acidradicals containing 1-10 carbon atoms, and at least one of R₁, R₂ and R₃is an alcoholic residue..]. .[.15. The lubricated container stock ofclaim 14 wherein the citric acid ester lubricant is present in an amountof about 0.15-0.40 gram for each 62,720 square inches of lubricatedsurface area..]. .[.16. The lubricated container stock of claim 14wherein R₁, R₂ and R₃ are alcoholic residua containing 1-4 carbonatoms..]. .[.17. The lubricated container stock of claim 14 wherein R₁,R₂ and R₃ are alcoholic residua containing 1-4 carbon atoms, and R₄ isselected from the group consisting of hydrogen and carboxylic acidradicals containing 1-4 carbon atoms..]. .[.18. The lubricated containerstock of claim 14 wherein the citric acid ester lubricant comprises atleast one substance selected from the group consisting of triethylcitrate, acetyl triethyl citrate, tributyl citrate, acetyl tributylcitrate, acetyl tri-2-ethylhexyl citrate and admixtures thereof..]..[.19. The lubricated container stock of claim 14 wherein the citricacid ester lubricant comprises acetyl tributyl citrate..]. .[.20. Thelubricated container stock of claim 19 wherein the citric acid esterlubricant is present in an amount of about 0.15-0.40 gram for each62,720 square inches of lubricated surface area..].
 21. In a method ofapplying an organic coating to metallic container stock wherein a layerof an initially fluid organic varnish, lacquer or enamel for thecontainer stock is applied to at least a portion of the surface area ofthe container stock and thereafter the layer of the initially fluidorganic varnish, lacquer or enamel is hardened to form the said organiccoating, the improvement which comprises improving the wettability ofthe said surface area of the container stock by the initially fluidorganic varnish, lacquer or enamel by .[.employing thereon as the saidlubricant a substantially uniform film.]. .Iadd.applying to the surfacearea of the container stock only a substantially uniform film of alubricant .Iaddend.consisting essentially of a citric acid ester of analcohol containing 1-10 carbon atoms, the citric acid ester beingpresent in an amount of about 0.05-1.0 gram for each 62,720 squareinches of lubricated surface area of the container stock.
 22. The methodof claim 21 wherein the citric acid ester lubricant is present in anamount of about 0.15-0.40 gram for each 62,720 square inches oflubricated surface area.
 23. The method of claim 21 wherein the metalliccontainer stock is selected from the group consisting of tinplate,blackplate, blackplate having a chromium-containing coating thereon, andmetallic aluminum of container stock gauge.
 24. The method of claim 23wherein the container stock is blackplate.
 25. The method of claim 23wherein the container stock is blackplate having a chromium-containingcoating thereon.
 26. The method of claim 23 wherein the container stockis metallic aluminum of container stock gauge.
 27. The method of claim23 wherein the container stock is tinplate.
 28. The method of claim 21wherein the citric acid ester lubricant has the structural formula:##STR5## wherein R₁, R₂ and R₃ are individually selected from the groupconsisting of hydrogen and alcoholic residua containing 1-10 carbonatoms, R₄ is selected from the group consisting of hydroben andcarboxylic acid radicals containing 1-10 carbon atoms, and at least oneof R₁, R₂ and R₃ is an alcoholic residue.
 29. The method of claim 28wherein R₁, R₂ and R₃ are alcoholic residua containing 1-4 carbon atoms.30. The coated metallic container stock prepared by the method of claim21.
 31. The method of claim 21 wherein the citric acid ester lubricant.[.comprises.]. .Iadd.consists of .Iaddend.at least one substanceselected from the group consisting of triethyl citrate, acetyl triethylcitrate, tributyl citrate, acetyl tributyl citrate, acetyltri-2-ethylhexyl citrate, and admixtures thereof.
 32. The method ofclaim 21 wherein the citric acid ester lubricant .[.comprises.]..Iadd.consists of .Iaddend.acetyl tributyl citrate.
 33. The method ofclaim 21 wherein the citric acid ester .[.comprises.]. .Iadd.consists of.Iaddend.acetyl tributyl citrate and is applied in an amount of about0.15-0.40 gram for each 62,720 square inches of lubricated surface area.. The method of claim 21 wherein the citric acid ester is of an alcoholcontaining 1-4 carbon atoms.
 35. The method of claim 23 wherein thecontainer stock is tinplate and the citric acid ester lubricant has thestructural formula: ##STR6## wherein R₁, R₂ and R₃ are individuallyselected from the group consisting of hydrogen and alcoholic residuacontaining 1-10 carbon atoms, R₄ is selected from the group consistingof hydrogen and carboxylic acid radicals containing 1-10 carbon atoms,and at least one of R₁, R₂ and R₃ is an alcoholic residue.
 36. Themethod of claim 35 wherein the citric acid ester lubricant is present inan amount of about 0.15-0.40 gram for each 62,720 square inches oflubricated surface area.
 37. The method of claim 35 wherein R₁, R₂ andR₃ are alcoholic residua containing 1-4 carbon atoms.
 38. The method ofclaim 35 wherein R₁, R₂ and R₃ are alcoholic residua containing 1-4carbon atoms, and R₄ is selected from the group consisting of hydrogenand carboxylic acid radicals containing 1-4 carbon atoms.
 39. The methodof claim 35 wherein the citric acid ester lubricant comprises at leastone substance selected from the group consisting of triethyl citrate,acetyl triethyl citrate, tributyl citrate, acetyl tributyl citrate,acetyl tri-2-ethylhexyl citrate, and admixtures thereof.
 0. The methodof claim 35 wherein the citric acid ester lubricant comprises acetyltributyl citrate.